Table of Contents

Thermal comfort parameters

Many subjective perceptions determine living comfort, even the colour of the surroundings plays a certain role – particularly for the mood of person who thereby expresses his or her perceptions. Living comfort mainly depends on the “thermal comfort”. This has been well-researched and the results have been incorporated into international standards (DIN ISO 7730). A large part of the information available to us today is due to the work of the Danish scientist P. O. Fanger (Wikipedia Seite).

Fig. 1 Air movement near to a Passive House window: Due to the small temperature difference between window surface and room air, the speed of air sinking at the window is small. At the floor, approximately 10 cm horizontally from the passive house window (U=0.8 W/(m²K)) the maximum air speed is a barely noticeable 0.11 m/s. If the insulating value of the window is worse, then air speed rises to disturbingly high values. Therefore it is recommended, with “normal windows”, to position a heating element under the window.(CFD simulation: J. Schnieders, PHI)

Optimal thermal comfort is established when the heat released by the human body is in equilibrium with its heat production. Fanger's comfort equation is derived from this fact. It creates a relationship between the activity (e.g. sleeping, running…) and clothing as well as the determining factors for the thermal surroundings, which are as follows:

air temperature

the temperature of the surrounding surfaces, this can also be summarised as the “radiant temperature”,

air speed and turbulence

air humidity.

There is a complete range of combinations of these four comfort factors where the level of comfort is very good, this is known as the comfort range. It can be determined by Fanger’s equation, documented in ISO 7730. Furthermore, according to this standard it is essential that

the sultriness limit in relation to the air humidity is not exceeded,

air speeds are within closely defined limits (for speeds under 0.08 m/s, the number of dissatisfied due to draughts is less than 6%)

the difference between radiant temperature and air temperature remains small,

the difference in the radiant temperature in various directions remains small (less than 5 °C, known as the “radiation temperature asymmetry”),

the indoor air temperature stratification is less than 2 °C between the head and ankles of a seated person,

the perceived temperatures in the room change by no more than 0.8 °C at different locations.

Regarding the last point, P.O. Fanger writes: “the more irregular the thermal field in a room is, the greater the expected number of dissatisfied people.”

What does this have to do with the Passive House?

Fig. 2 Stratification: The temperature stratification of air is also imperceptible with the passive house window. Therefore the heating element can be placed also at an inner wall while still reaching optimal comfort in accordance with ASHRAE Comfortclass “A”. (computation: J. Schnieders, PHI).

Fig. 3 Passive House Practice: Infrared image of the interior side of a passive house window. All surfaces (wall structure, window frame, and the glazing) are pleasantly warm (over 17 °C). Even at the glass edge, the temperature doesn't fall below 15 °C (light-green portion)
(Source: PHI, from the Passive House Kranichstein).

Fig. 4 Typical Practice: For comparison, here is a typical old building's window. The center of glass surface temperature is already below 14 °C. In addition, there are large installation thermal bridges, particularly where the window meets the concrete wall. The consequences: significant eadiant temperature asymmetry, drafts, and pooling of cold air in the room.
(IR-fotography: PHI, image taken at the office of PHI)

Fig. 5 low-e glazing (2 panes) already have higher surface temperatures (16 °C in the average). Additionally, the poor insulation of the conventional window frame is remarkable. Passive house frames allow a dramatic improvement in insulating quality. (IR fotography: PHI, in the corridor of the institute)

It is exciting that by the requirements of the passive house standard all comfort criteria are automatically optimally fulfilled - substantially improving the thermal insulation simultaneously improves thermal comfort. This can be understood as follows:

By improving thermal insulation (regardless of which external building building component, e.g. wall, roof, floor, etc.) the heat flow from the inside to the outside is reduced.

Therefore the heat flow of the room area to the interior surface of this external building component is also reduced. That heat flow has to flow through the thermal film coefficinet of the surface (radiation and convection).

The smaller heat flow implies a resultant smaller drop of temperature over this thermal resistance. In other words:

There is a smaller temperature difference between the room area (the surfaces in the area and the room air) and the interior surface of the well insulated building component.

The practical consequence: With highly insulating external construction components, the temperature of the interior surface is only slightly different from the other temperatures in the area; this applies both in summer and winter. In the winter, the interior surfaces of the external construction components are comfortably warm (external walls, roofs etc. at the most 1 °C under the ambient temperature, window surfaces maximally 3 to 3.5 °C under it [ 1 ]). The definition of “Passive House quality” windows is straightforward: The insulating efficiency of a window suitable for Passive Houses must be so good that under the coldest design conditions, the equation:

θ area - θ Oberfl ≤ 3.5 °C

still holds true. These small temperature differences have the following affects on the comfort criteria:

Air speeds in the area (apart from leaks) are due to free convection at cooler surfaces. Due to the small temperature differences the convective currents, and consequently air speeds, are now very small. Fig. 1 in the left column shows a CFD (Computational Fluid Dynamic) simulation result: There is no draft in the room, even without a heating element under the window.

If the external surface temperature is not more than 3.5 °C below the ambient temperature, then the radiant temperature difference in different directions cannot be greater than 3.5 °C. The thermography photographs in Fig. 3 to Fig. 5 show the difference between windows of various insulating quality.

The room air temperature stratification between the head and feet of a sitting person is less than 2 °C - however only under the condition that the effective median U-value of the external building components is under 0.85 W/(m²K). See Fig. 2 in the left column.

The percieved temperature varies less than 0.8 °C within the area.

Because all of the comfort criteria are fulfilled, without the need for a compensatory radiation heating surface, everythere in the Passive House there is “automatically” a radiant heat climate, independently of how the heat is supplied. More still: since there are no large temperature differences, air movement remains small. The results represented here are documented in the publication [ 1 ].

The comfort characteristics of well insulated buildings can be observed in practice and is confirmed by three independent research results: